This invention relates generally to the atomization of fluids and more particularly to the high volume, low pressure atomization and entrainment of liquids, such as paint, for application to a surface.
Two types of apparatus for the atomization and particulation of fluids are currently in use, namely pneumatic and airless. As they refer to application of paint type coatings, both types normally comprise high pressure apparatus. However, at least one type of low pressure system is also known and comprises the system disclosed in a recently issued patent granted to the present inventor on July 25, 1989, namely U.S. Pat. No. 4,850,809 entitled, "Air Operated Low Pressure Spraying System". This patent will be discussed hereinafter and is specifically meant to be incorporated herein by reference.
Conventional high pressure, low volume pneumatic systems have the disadvantage of locally producing a wasteful cloud of fluid and air commonly referred to as "overspray". The solvent vapors become part of the atmosphere and present a hazard not only to the environment and user, but also to the atomizing equipment. The visible components of overspray comprise solids entrapped in the vapor which results not only in the degrading of the quality of the work surface, but also in the contamination of the work site. High pressure, airless systems also generate a type of overspray referred to as "bounceback". This condition is created by the high velocity by which the fluid is propelled to the surface. The rebounding particles, solids and vapors, form a cloud similar to the overspray generated by pneumatic apparatus. Both types of systems, however, produce the same undesirable results.
Overspray and its reduction has become a subject of major concern to industries involved in the atomization of fluids. An amendment to the "Clean Air Act" of February, 1987 as it relates to hydrocarbon emission controls, established limitations and standards of performance for fluid transfer. Those industries affected are manufacturers and end users of commercial and consumer solvents, architectural coatings, pesticides, and all apparatus and methods involved in their application. Particular emphasis is being placed on government and military users. Additionally, individual states are implementing this act with their own pollution control bills. In some states, high pressure paint systems and adaptations that rely on high pressure, low volume application of atomization are being studied for restricted use. This could have a disastrous effect on thousands of small businesses. Manufacturers of fluids, in order to reduce the percentage of carrier solvents, are now required to increase their solids content and change fluid chemistry. This places a new burden on the atomizing system to atomize these high solids and viscosities.
All spraying systems require some type of apparatus to atomize the fluid and deliver it to the work surface. This apparatus is commonly called a spray gun. Guns vary in their configuration, size, weight and internal composition. Most attempts at improved fluid atomization for the purposes of spray painting have been, moreover, directed to the gun. One such spray gun, as disclosed in U.S. Pat. No. 3,796,376, granted to I. O. M. Farnsteiner on Mar. 12, 1974, for example, locates a jet venturi induction pump in the handle of the gun. Its failure to achieve commercial acceptability is due to the location of the jet venturi induction pump. Its function was to convert high pressure, low volume shop air to low pressure, high volume air. However, because of its close proximity to the work area, the ambient air drawn into the device was contaminated by overspray. Therefore the jet venturi induction pump was continuously introducing contaminated air into its internals. As a result, this contaminated air left deposits on the internal passages and orifices of the apparatus causing it to malfunction. Additionally, the user's hand could easily block the induction ports preventing a continuous inflow of ambient air. The entire system, as a result, was dominated by shop air. Lastly, the type of apparatus cannot be adjusted to meet varying fluid viscosities. All these adverse conditions negated the role of the device as an acceptable improvement. To successfully atomize conventionally, pressures of 50 to 60 psig at 4 to 8 cfm. are required. The gun is designed to atomize fluids by the violent forward motion of the air as its exits the nozzle. Because the air nozzle is considerably larger than the fluid nozzle, it delivers more air than is necessary. The explosion into the atmosphere results in overspray. Thus, there is a direct relationship between overspray and high pressure.
This led to the development of the high volume, low pressure system disclosed in the present inventor's above referenced U.S. Pat. No. 4,850,809 wherein a portable enclosure is used to house a variable jet venturi induction pump which physically separates the induction pump from the area of application and other potentially contaminating elements. Ambient air remote from the work site is introduced into the portable enclosure after regulation and filtration. A low pressure field for driving a spray gun is generated by the induction pump by increasing the velocity of the air fed into a venturi section of the pump which is free of contaminates generated, for example, by overspray in the vicinity of the area of application.